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From: Bodo Eggert <7eggert@elstempel.de>
Subject: Re: IS_ERR Threshold Value
To: Jan Engelhardt <jengelh@linux01.gwdg.de>,
       Andreas Mohr <andi@rhlx01.fht-esslingen.de>,
       Ralf Baechle <ralf@linux-mips.org>,
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       linux-kernel@vger.kernel.org
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Date: Thu, 19 Oct 2006 01:29:29 +0200
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Jan Engelhardt <jengelh@linux01.gwdg.de> wrote:

>>> +#define IS_ERR_VALUE(x) unlikely((x) >= (unsigned long)-MAX_ERRNO)
> 
> There seems to be a slight problem with doing that. Running
> `ldd /bin/bash` prints out
> 
> linux-gate.so.1 =>  (0xffffe000)
> 
> and the topmost address a kernel function can return is 0xFFFFf000 when
> MAX_ERRNO=4095, but that is going to be tight with the vdso mapped at
> 0xffffE000.

http://www.trilithium.com/johan/2005/08/linux-gate/

"... an x86 box where processes live in plain old 32-bit address spaces
divided into pages of 4096 bytes, making ffffe000 the penultimate page.
The very last page is reserved to catch accesses through invalid pointers,
e.g. dereferencing a decremented NULL pointer or a MAP_FAILED pointer
returned from mmap."

Therefore, MAX_ERRNO < PAGE_SIZE is safe.
-- 
Ich danke GMX dafür, die Verwendung meiner Adressen mittels per SPF
verbreiteten Lügen zu sabotieren.

http://david.woodhou.se/why-not-spf.html